Rejuvinating a Feather Pillow
What you can do depends upon the condition of the cover and of the down & feathers inside. Are the feathers lumpy or have they just lost their fluffiness? Are you wanting the cushions to more fluffy, or to be more supportive. One problem with feather sofa or chair cushions is they might not have enough body to them and so you can just sit right through them.
To fluff them up you can either clean them, add to them, or replace everything. First, check the condition of the ticking cover, see if the seams are are tight or if any feathers are coming through. Make a new ticking cover or fix any holes in the old ticking before you clean them, you can take them to a dry cleaner. Or if you decide to wash them, put two of them in the load to counterbalace them. (This only applies if the cushions are small. If the cushions are large and thick, washing them might be a good idea as they might just get soggy in the middle and might take a lot to get them to dry out in the middle). In the drier put a couple of clean tennis shoes, which will help fluff them and break up the clumps. Also, make sure you dry them completely.
If they don't need to be cleaned, then you may just need to add more feathers. If you add more feathers you can just use some of your feather pillows, or you can go purchase some feather pillows. You can also purchase more down/feathers from an upholstery supply outfit, but you usually have to purchase a large bag of them, which is pretty expensive. That's why I suggest purchase feather pillows. YOu can often get them much cheaper than buying just the feathers.
Another way to add more body to a feather pillow is to add a foam core to the middle of the feather pillow. you can go to Down Feather with Foam Insert., which will show you how a foam core feather pillow is constructed.
Another possibility, if the feather pillows are quite old, is that the feather might have just disintegrated. As an example, I remember back some years ago when my wife wanted me to recover her favorite feather pillow, which she had since childhood. When I opened it up the feathers inside had completely disintegrated into a blackish yucky mess. I ended up making everything new for her. Down and feathers, like anything else, don't last forever. If the cushions get in too bad of shape, sometimes it's just better to make new ticking covers and replace the filling with new feathers/down